Method and apparatus for forming copper indium gallium chalcogenide layers

ABSTRACT

A multilayer structure to form absorber layers for solar cells. The multilayer structure includes a base comprising a contact layer on a substrate layer, a first layer on the contact layer, and a metallic layer on the first layer. The first layer includes an indium-gallium-selenide film and the gallium to indium molar ratio of the indium-gallium-selenide film is in the range of 0 to 0.8. The metallic layer includes gallium and indium without selenium. Additional selenium is deposited onto the metallic layer before annealing the structure for forming an absorber.

This application claims priority to U.S. Provisional Application Ser.No. 60/983,045, filed Oct. 26, 2007, entitled “Method and Apparatus forForming Copper Indium Gallium Chalcogenide Layers”, which application isexpressly incorporated by reference herein.

The present invention relates to method and apparatus for preparing thinfilms of semiconductor films for radiation detector and photovoltaicapplications.

FIELD OF THE INVENTION Background

Solar cells are photovoltaic devices that convert sunlight directly intoelectrical power. The most common solar cell material is silicon, whichis in the form of single or polycrystalline wafers. However, the cost ofelectricity generated using silicon-based solar cells is higher than thecost of electricity generated by the more traditional methods.Therefore, since early 1970's there has been an effort to reduce cost ofsolar cells for terrestrial use. One way of reducing the cost of solarcells is to develop low-cost thin film growth techniques that candeposit solar-cell-quality absorber materials on large area substratesand to fabricate these devices using high-throughput, low-cost methods.

Group IBIIIAVIA compound semiconductors comprising some of the Group IB(copper or Cu, silver or Ag, gold or Au), Group IIIA (boron or B,aluminum or Al, gallium or Ga, indium or In, thallium or Tl) and GroupVIA (oxygen or 0, sulfur or S, selenium or Se, tellurium or Te, poloniumor Po) materials or elements of the periodic table are excellentabsorber materials for thin film solar cell structures. Especially,compounds of Cu, In, Ga, Se and S which are generally referred to asCIGS(S), or Cu(In,Ga)(S,Se)₂ or CuIn_(1-x)Ga, (S_(y)Se_(1-y))_(k), where0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed insolar cell structures that yielded conversion efficiencies approaching20%. Absorbers containing Group IIIA element Al and/or Group VIA elementTe also showed promise. Therefore, in summary, compounds containing: i)Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA,and iii) at least one of S, Se, and Te from Group VIA, are of greatinterest for solar cell applications. It should be noted that althoughthe chemical formula for CIGS(S) is often written as Cu(In,Ga)(S,Se)₂, amore accurate formula for the compound is Cu(In,Ga)(S,Se)_(k), where kis typically close to 2 but may not be exactly 2. For simplicity we willcontinue to use the value of k as 2. It should be further noted that thenotation “Cu(X,Y)” in the chemical formula means all chemicalcompositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). Forexample, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly,Cu(In,Ga)(S,Se)₂ means the whole family of compounds with Ga/(Ga+In)molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from0 to 1.

The structure of a conventional Group IBIIIAVIA compound photovoltaiccell such as a Cu(In,Ga,Al)(S,Se,Te)₂ thin film solar cell is shown inFIG. 1. The device 10 is fabricated on a substrate 11, such as a sheetof glass, a sheet of metal, an insulating foil or web, or a conductivefoil or web. The absorber film 12, which comprises a material in thefamily of Cu(In,Ga,Al)(S,Se,Te)₂, is grown over a conductive layer 13 orcontact layer, which is previously deposited on the substrate 11 andwhich acts as the electrical contact to the device. The substrate 11 andthe conductive layer 13 form a base 13A on which the absorber film 12 isformed. Various conductive layers comprising Mo, Ta, W, Ti, and theirnitrides etc. have been used in the solar cell structure of FIG. 1. Ifthe substrate itself is a properly selected conductive material, it ispossible not to use the conductive layer 13, since the substrate 11 maythen be used as the ohmic contact to the device. After the absorber film12 is grown, a transparent layer 14 such as a CdS, ZnO, CdS/ZnO orCdS/ZnO/ITO etc. stack is formed on the absorber film 12. Radiation 15enters the device through the transparent layer 14. Metallic grids (notshown) may also be deposited over the transparent layer 14 to reduce theeffective series resistance of the device. The preferred electrical typeof the absorber film 12 is p-type, and the preferred electrical type ofthe transparent layer 14 is n-type. However, an n-type absorber and ap-type window layer can also be utilized. The preferred device structureof FIG. 1 is called a “substrate-type” structure. A “superstrate-type”structure can also be constructed by depositing a transparent conductivelayer on a transparent superstrate such as glass or transparentpolymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)₂ absorberfilm, and finally forming an ohmic contact to the device by a conductivelayer. In this superstrate structure light enters the device from thetransparent superstrate side. A variety of materials, deposited by avariety of methods, can be used to provide the various layers of thedevice shown in FIG. 1.

In a thin film solar cell employing a Group IBIIIAVIA compound absorber,the cell efficiency is a strong function of the molar ratio of IB/IIIA.If there are more than one Group IIIA materials in the composition, therelative amounts or molar ratios of these IIIA elements also affect theproperties. For a Cu(In,Ga)(S,Se)₂ absorber layer, for example, theefficiency of the device is a function of the molar ratio of Cu/(In+Ga). Furthermore, some of the important parameters of the cell, such asits open circuit voltage, short circuit current and fill factor varywith the molar ratio of the IIIA elements, i.e. the Ga/(Ga+In) molarratio. In general, for good device performance Cu/(In +Ga) molar ratiois kept at around or below 1.0. As the Ga/(Ga+In) molar ratio increases,on the other hand, the optical bandgap of the absorber layer increasesand therefore the open circuit voltage of the solar cell increases whilethe short circuit current typically may decrease. So far experimentalresults have shown that a Ga/(Ga+In) ratio of about 0.2-0.3 at thejunction area (top 0.1 to 0.3 μm of the CIGS surface) yields the highestefficiency solar cells. When this ratio increases further, the deviceefficiency gets reduced. Although the reasons for this are not fullyunderstood, it is reported that the electronic quality of CIGS materialgets worse as the Ga/(Ga+In) ratio increases beyond 0.3. It is importantfor a thin film deposition process to have the capability of controllingboth the molar ratio of IB/IIIA, and the molar ratios of the Group IIIAcomponents in the composition.

One attractive technique for growing Cu(In,Ga)(S,Se)₂ type compound thinfilms for solar cell applications is a two-stage process where metalliccomponents of the Cu(In,Ga)(S,Se)₂ material are first deposited onto asubstrate during the first stage of the process, and then reacted with Sand/or Se in a high temperature annealing process during the secondstage. Sputtering and evaporation techniques have been used in prior artapproaches to deposit the layers containing the Group IB and Group IIIAcomponents of the precursor stacks during the first stage of such aprocess. In the case of CuInSe₂ growth, for example, Cu and In layerswere sequentially sputter-deposited on a substrate and then the stackedfilm was heated in the presence of gas containing Se at elevatedtemperature for times typically longer than about 30 minutes, asdescribed in U.S. Pat. No. 4,798,660. More recently U.S. Pat. No.6,048,442 disclosed a method comprising sputter-depositing a stackedprecursor film comprising a Cu—Ga alloy layer and an In layer to form aCu—Ga/In stack on a metallic back electrode layer and then reacting thisprecursor stack film with one of Se and S to form the absorber layer.U.S. Pat. No. 6,092,669 described sputtering-based equipment forproducing such absorber layers. According to a method described in U.S.Pat. No. 4,581,108, a Cu layer is first electrodeposited on a substrate;this is then followed by electrodeposition of an In layer and heating ofthe deposited Cu/In stack in a reactive atmosphere containing Se to formCuInSe₂ or CIS. Although CIS formation using two-stage processes israther straight forward, complications arise when Ga is added to be ableto form a CIGS absorber.

Curve A in FIG. 2 schematically shows a typical distribution profile forthe Ga/(Ga+In) molar ratio for a Cu(In,Ga)Se₂ absorber layer formed by atwo-stage process involving selenization of metallic precursorscomprising Cu, In and Ga. As can be seen from this figure, one problemfaced with the selenization type or two-stage processes is thedifficulty to distribute Ga uniformly through the thickness of theabsorber layer formed after reaction of Cu, In and Ga containingmetallic precursor film with Se. It is believed that when a metallicprecursor film comprising Cu, In and Ga is deposited first on a contactlayer of a base and then reacted with Se, the Ga-rich phases segregateto the film/base interface (or the film/contact layer interface) becausereactions between Ga-bearing species and Se are slower than thereactions between In-bearing species and Se. Therefore, such a processyields compound absorber layers with surfaces that are rich in In andpoor in Ga. Various reports in the literature have described thisphenomenon (see for example, Basol et al., Progress in Photovoltaics,vol. 8 (2000) p. 227, Alberts et al., Solar Energy Materials and SolarCells, vol. 64 (2000) p. 371, Marudachalam et al., J. Appl. Phys., vol.82 (1997) p. 2896, Delsol et al., Solar Energy Materials and SolarCells, vol. 82 (2004) p. 587).

When a solar cell is fabricated on an absorber layer with Ga gradationsuch as the one shown in FIG. 2, the active junction of the device isformed within the surface region with a low Ga/(Ga+In) ratio, which isin fact zero for Curve A. This surface portion or region, therefore, ispractically a CuInSe₂ layer with a small bandgap and consequently solarcells fabricated on such layers display low open circuit voltages(typically in the range of 400-500 mV) and thus lower efficiencies. Incontrast, curve B in FIG. 2 schematically shows a relatively uniformGa/(Ga+In) molar ratio distribution through the thickness of theabsorber. Solar cells fabricated on such absorbers display highervoltage values of typically over 600 mV due to the presence of Ga(typically 20-30%) near the surface region. The world record holdingCIGS solar cells with over 19% conversion efficiency were demonstratedon such an absorber obtained by a co-evaporation process (see, forexample Ramanathan et al., Progress in Photovoltaics, vol. 11 (2003) p.225). Obtaining Ga distribution profiles with more Ga near the surfaceregion for absorbers fabricated using two-stage processes is importantto increase the performance of such absorbers.

As described above, the co-evaporation methods where the Cu, In, Ga andSe species are co-deposited onto a surface of a heated substrate wherethey react and form the compound monolayer at a time have the capabilityto control and shape the distribution of Ga and In through the CIGS film(see for example, U.S. Pat. Nos. 5,356,839, 5,436,204, and 5,441,897).Although attractive for manufacturing, the two-stage processes have nothad this capability because the film deposition step, when the Cu, In,Ga and possibly Se species are deposited in a non-reactive manner, isseparated from the reaction step when the actual CIGS compound film isfully formed with properties appropriate for solar cell fabrication.Several attempts were made to investigate the possibility of controllingGa distribution within absorbers grown by the two-stage processes.Marudachalam et al. (J. Appl. Phys., vol. 82 (1997) p. 2896), forexample, annealed CIGS layers at high temperatures to diffuse Ga to thesurface from the back side of the absorber after forming a CIGS layerwith Ga distribution similar to curve A in FIG. 2. Nakagawa et al.(14^(th) European Photovoltaic Solar Energy Conf., 1997, p. 1216)prepared CIGS layers using various types of precursor stacks includingmetallic and non-metallic layers with the goal of producing different Gadistribution profiles alter reacting the stacks. The stacks investigatedby Nakagawa et al. were In—Se/Cu/Ga—Se, In—Se/Ga—Se/Cu, Cu/In—Se/Ga—Se,Cu/Ga—Se/In—Se, Ga—Se/Cu/In—Se, Ga—Se/In—Se/Cu, In—Se/Ga—Se/Cu/Ga—Se,Ga—Se/In—Se/Cu/Ga—Se, and Ga—Se/In—Se/Ga—Se/Cu stacks, where In—Se andGa—Se refer to selenides of In and Ga, respectively.

As the review above demonstrates there is still need to develop twostage processing approaches that can yield desirable Ga distributionprofiles in CIGS type absorber layers so that high efficiency solarcells may be fabricated using such absorber layers.

SUMMARY OF THE INVENTION

Present invention provides a method of making a multilayer structure formanufacturing solar cell absorbers. The multilayer structure may bebuilt on a continuous flexible foil or workpiece which is suitable forroll-to-roll or reel-to-reel manufacturing processes.

In an aspect of the present invention, a multilayer structure to formabsorber layers for solar cells is provided. The multilayer structureincludes a base having a substrate layer; a first layer formed on thebase, and a metallic layer formed on the first layer. The first layerincludes an indium-gallium-selenide film, which the gallium to indiummolar ratio of the indium-gallium-selenide film is in the range of 0 to0.8. The metallic layer includes gallium and indium without a Group VIAmaterial, and indium and gallium in the metallic layer form a stackcomprising at least one indium film and at least one gallium film. Amolar ratio of gallium to gallium and indium in the metal layer is inthe range of 0.2-0.3.

In another aspect of the present invention, a process of forming a GroupIBIIIAVIA absorber layer on a base is provided. The process includesforming a first layer comprising an indium-gallium-selenide compoundfilm on the base, forming a metallic layer on the first layer, themetallic layer comprising a Group IB metal, a Group IIIA metal andanother Group IIIA metal without a Group VIA material, and reacting thefirst layer, the metallic layer and a Group VIA material. The firstlayer further includes a first metal film of a Group IB metal, whereinthe indium-gallium-selenide compound material film is deposited over thefirst metal film. Forming the metallic layer includes depositing acopper film onto the first layer, depositing a gallium film onto thecopper film, and depositing an indium film onto the gallium film.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects and features of the present invention willbecome apparent to those of ordinary skill in the art upon review of thefollowing description of specific embodiments of the invention inconjunction with the accompanying figures, wherein:

FIG. 1 is a cross-sectional view of a solar cell employing a GroupIBIIIAVIA absorber layer.

FIG. 2 shows Ga/(Ga+In) molar ratios in two different CIGS absorberlayers, one with a Ga-poor surface (curve A) and the other with a moreuniform Ga distribution (curve B).

FIG. 3 schematically shows the effect of changing the absorber thicknesson the distribution of Ga.

FIG. 4 schematically shows the effect of changing the total Ga contenton the Ga distribution in the resulting CIGS absorber after reactionwith Group VIA material.

FIG. 5 schematically shows various Ga distribution profiles providedaccording to an embodiment of the present invention.

FIG. 6A is a structure according to an embodiment of the presentinvention.

FIG. 6B is another structure according to an embodiment of the presentinvention.

DETAILED DESCRIPTION

Present invention provides a method of making a multilayer structure formanufacturing solar cell absorbers. The multilayer structure may bebuilt on a continuous flexible foil or workpiece which is suitable forroll-to-roll or reel-to-reel manufacturing processes, i.e., feeding thecontinuous substrate from a supply roll into the process tool thatmanufactures the multilayer structure and taking up and wrapping thefinished product around a take-up roll.

In a first embodiment the method includes forming a first layer of aprecursor stack on a conductive contact layer of the continuousworkpiece which may be a metallic foil. The first layer may include agallium-indium-selenide compound layer with a gallium to indium ratio of0-0.8. On the first layer, a metallic layer including copper, galliumand indium metals is formed. In the metallic layer, gallium to indiumratio may be in the range of 0.2-0.3. After preparing the precursorstack another layer of selenium may be added on top of the stack and themultilayer structure is reacted to form the Group IBIIIAVIA solar cellabsorber.

FIG. 3 schematically shows three different Ga distribution profilesthrough a CIGS absorber layer formed by selenization of a metallicprecursor layer comprising Cu, In and Ga. The exemplary metallicprecursor layer in this case may include but is not limited to stackssuch as Cu/Ga/In, Cu/Ga/Cu/In, and Cu/Ga/Cu/In/Ga deposited on a baseusing methods such as sputtering, evaporation or electrodeposition. Alayer of Se may then be deposited on top of the precursor stack forminga pre-absorber structure. The pre-absorber structure may then beconverted into a Cu(In,Ga)Se₂ or CIGS absorber layer by increasing thetemperature of the structure to a range of 400-600 C and facilitatingreaction between, Cu, In, Ga and Se. As discussed earlier, such aprocess yields a Ga/(Ga+In) ratio distribution such as the one shown inCurve I in FIG. 3. It should be noted that in this example the thicknessof the absorber is “t₁”, and “t₁” may be in the range of 1-1.5 μm, whichis a range that yields good quality solar cells in the laboratory.

As explained before, when a solar cell is fabricated on the surface ofan absorber represented by Curve I of FIG. 3, the junction sees anabsorber surface with Ga/(Ga+In) ratio of zero, i.e. it practically seesa CuInSe₂ or CIS surface. Therefore, the voltage of such a cell is low(typically in the range of 400-500 mV) and its efficiency is limiteddespite the fact that the overall Ga/(Ga+In) ratio integrated throughthe thickness “t₁” may be 20-30%, or even more.

One way of pushing Ga more towards the absorber surface during atwo-stage process, while keeping the overall Ga/(Ga+In) ratio in theabsorber the same, is reducing the thickness of the absorber layerformed, while keeping the Ga/(Ga+In) ratio the same. Curves II and IIIschematically show how the Ga distribution may change through the formedabsorber layer as its thickness is reduced to “t₂” and “t₃”,respectively. In this example, “t₂” may be in the range of 0.5-0.75 μmand “t₃” may be in the range of 0.2-0.4 μm. The problem with such anapproach is the fact that as the thickness of the absorber gets reducedto and below 0.75 μm, light absorption gets reduced and mechanicaldefects such as pinholes introduce shunts in devices fabricated on suchthin layers. As a result, even though Ga is pushed to the surface, theoverall efficiency of the solar cells may actually go down.

Another method that may be used to increase the amount of Ga near thesurface region of a CIGS layer prepared by a two-stage process is toincrease the overall Ga content of the precursor film and the absorberlayer beyond the optimum 20-30%. The Ga/(Ga+In) ratio plots in FIG. 4show schematically how the Ga content near the absorber surface may beincreased towards the optimum level of 20-30% as the total Ga content ofthe film is increased. In this case the thickness of the absorber iskept at a level (such as 1-1.5 μm) that yields films with goodmechanical integrity and low pinhole density. The overall Ga/(Ga+In)ratio in the metallic precursor stack may be increased from 30% (Curve“X”) to 50% (Curve “Y”) and 70% (Curve “Z”). As can be seen, the Gamolar ratio near the absorber surface may be increased this way,eventually getting close to the ideal 20-30% range. The problem withthis approach, however, may be the fact that although the near surfaceregion may have the good solar cell material necessary for highefficiency, most of the absorber layer contains a Ga/(Ga+In) ratio thatis much higher than that. In fact the absorber/contact layer interfacecontains essentially a CGS or CuGaSe₂ layer. As explained earlier, theelectronic quality of CIGS layers with Ga molar ratio of higher than 30%is not good. Therefore, generated current collection from these regionsmay not be efficient. As a result, although the voltage of the cell maybe high (higher than 600 mV) due to the presence of Ga near the surfaceregion, the short circuit current density may go down due to presence ofhigh Ga deeper in the absorber, thus lowering the overall solar cellefficiency. Using this approach, however, it is possible to fabricatesolar cells with efficiency values over 10%.

To address the issues described above another embodiment of a two-stagemethod of the present invention may be used. In this method, anon-metallic film is used in a first layer of a precursor stack.Accordingly, a portion of the indium necessary for absorber formation isincluded in the precursor material in the form of a non-metallicmaterial, such as indium-Group VIA or indium-gallium-Group VIA compound,for example, indium-selenide or indium-gallium-selenide. Thenon-metallic material film is placed near the contact layer interfaceaway from the surface region to form the first layer of a precursorstack or structure. There can be one or more metal films such as acopper film, between the contact layer and the film comprising thenon-metallic material. Over the first layer, a metallic stack or layeris built. Gallium, In and optionally Cu are included in the metalliclayer in metallic form so that during the reaction step of the two-stageprocess fast diffusion and grain fusing may take place between the firstlayer and the metallic layer resulting a columnar grain structure in theformed CIGS compound layer.

It should be noted that in a columnar structure, grains extend from thesurface of the compound film all the way to the contact layer. In thisapproach presence of metallic Ga near the surface region of the filmallows good grain growth and at the same time Ga is encouraged to reactwith the Group VIA material and stay near the surface region. Theprecursor stack of the present invention may be configured in variousstructures. For example, a precursor stack ofcopper/indium-gallium-selenide/metal layer (Cu/(In,Ga)Se/metal layer),or (In,Ga)Se/metal layer may be deposited on a base, forming abase/Cu/(In,Ga)Se/metal layer, or base/(In,Ga)Se/metal layer structureIn these precursor structures, the (In,Ga)Se layer is an indium-galliumselenide layer with Ga/(Ga+In) ratio ranging from 0 to about 0.8. Themetal layer comprises Ga and In and optionally Cu. These precursorstacks are depicted in FIGS. 6A and 6B. In FIG. 6A the base 13Acomprises a substrate 11 and a contact layer 13, and the precursor stack100 is deposited on the exposed surface of the contact layer. In thisembodiment, the precursor stack 100 comprises a first layer having a Cufilm 110 and an (In,Ga)Se film 120, and a metal layer 130. The metallayer 130 may or may not comprise Cu.

The metal layer 130 within the precursor stack 100 may itself be a stackof metal films such as a Ga/In, In/Ga, Cu/Ga/In, Cu/In/Ga, Ga/In/Cu,In/Ga/Cu, Cu/Ga/In, Cu/In/Ga, Cu/Ga/In/Cu, and Cu/In/Ga/Cu stack and thelike. It is also possible that the metals (In, Ga and optionally Cu)within the metal layer 130 be in the form of mixtures or alloys ratherthan discrete layers forming a stack. In FIG. 6B the base 13A comprisesa substrate 11 and a contact layer 13, and the precursor stack 200 isdeposited on the exposed surface of the contact layer. The precursorstack 200 comprises a first layer comprising an (In,Ga)Se film 210, anda metal layer 220. In this case the metal layer 220 has to contain allof Cu, In and Ga. The metal layer 220 within the precursor stack 200 mayitself be a stack of metal films such as a Cu/Ga/In, Cu/In/Ga, Ga/In/Cu,In/Ga/Cu, Cu/Ga/In, Cu/In/Ga, Cu/Ga/In/Cu, and Cu/In/Ga/Cu stack and thelike. It is also possible that the metals (In, Ga and Cu) within themetal layer 220 be in the form of mixtures or alloys rather thandiscrete layers forming a stack.

It should be noted that the precursor stacks of the present inventionare different than the stacks utilized by Nakagawa et al. reference. InNakagawa's work Ga in the precursor is chemically tied in a Ga-selenidelayer. In the present stack it is important that the Ga is placed nearthe surface of the precursor stack and is in metallic state either byitself or alloyed with another metal. The metal film or metal layer ofthe present invention also comprises In, unlike that of Nakagawareference. Presence of such metallic phases with low meltingtemperatures (Ga melting temperature around 30° C. and In meltingtemperature about 156° C.) in the precursor stack assures good fusingand grain growth during the reaction or selenization step. It is ofcourse possible to add some amount of Group VIA material within themetal layer 130 of FIG. 6A and the metal layer 220 of FIG. 6B as long asthe metallic character and low temperature melting character is kept.Therefore, the amount of Group VIA material (such as Se) added to withinsuch metal layers should be <30% atomic, preferably <10% atomic of thetotal Cu, In, Ga and Group VIA material within the metal layer.

Once the above described base/Cu/(In,Ga)Se/metal film, orbase/(In,Ga)Se/metal layer structures depicted in FIGS. 6A and 6B areformed (as stacks that have substantially distinct layers without anysubstantial reaction between them as depicted in FIGS. 6A and 6B), thestacks may be selenized (and thus causing a reaction between thedistinct layers as also noted below) to convert them into absorberlayers. Selenization may be carried out at temperatures in the range of400-600° C. in Se-containing atmosphere for times ranging from 5 to 60minutes. A Se layer may also be deposited on the exposed surface of theprecursor stack of FIG. 6A or FIG. 6B before the reaction step, in whichcase rapid reaction of Cu, In, Ga and Se may be more readily achieved ina rapid thermal annealing process.

The benefit of the unique precursor stacks of the present invention maybe understood in reference to FIG. 5. Curve “α” in FIG. 5 schematicallyshows the Ga distribution through an absorber layer formed byselenization of a Cu/(In)Se/metal film stack of FIG. 6A where the metalfilm may comprise Ga and In. Similar results may be obtained if theprecursor stack is (In)Se/metal layer (see FIG. 6B), where, in thiscase, the metal layer may comprise Ga, In and Cu. In this example theGa/(Ga+In) ratio within the metal film is selected to be about 0.3 sincewhen selenized, the metal film will be converted into an absorberportion near the surface with the desired Ga/(Ga+In) ratio of about0.2-0.3.

The thickness of the Cu deposited on the base or included in the metalfilm, on the other hand, is selected so that it is adequate to convertthe (In)Se layer into a substantially CIS layer and the metal film intosubstantially a CIGS surface layer after the selenization step. Copperis very mobile and may be placed on the contact layer, under the (In)Selayer, or alternately it may be included in the metal layer withoutchanging results. In any case, as can be seen from FIG. 5, Curve “α”, aGa-rich surface may be formed using this approach even with a total filmthickness “t” of 1-1.5 μm. The reason for this is the fact that much ofthe In (such as 50-80%) necessary for the overall CIGS absorber layer ischemically tied in the form of indium selenide in the back of theprecursor stack and cannot readily react with additional seleniumintroduced during the selenization step. Therefore, it cannot diffuse tothe surface and replace Ga near the absorber surface during the reactionstep. Rather, it stays in the back of the device. The Ga distribution inthe back of the absorber may be controlled independently in the presentapproach by increasing or decreasing the Ga content of the (In,Ga)Selayer. Curve “α” is for the case with no Ga in the (In,Ga)Se layer, i.e.use of an (In)Se layer. As the amount of Ga is increased in the(In,Ga)Se layer, one can form Ga grading depicted by curves “β”, “γ”,and “θ”. To obtain an absorber represented by Curve “γ” for example, theGa/(Ga+In) ratio within the (In,Ga)Se layer may be around 0.4. Thisratio may be around 0.7 for the absorber represented by Curve “θ”.

The ability of being able to control the Ga content in the back of theabsorber without affecting much the Ga content within the surface layerof the absorber is valuable since both may be optimized separately. TheGa profile of Curve “θ”, for, example forms an electron reflector thatcan help light generated current collection. The Ga profile of Curve “γ”is similar to films grown by the co-evaporation method that yields veryhigh efficiency solar cells. The fact that, Ga can be brought to thesurface without reducing the thickness of the absorber layer to levelsbelow 0.5 μm, or increasing the overall Ga/(Ga+In) ratio within theabsorber to levels beyond 60-70% opens up the process window of thetwo-stage techniques for high efficiency solar cell manufacturing.

The layers within the precursor stack of the present invention may bedeposited by one or more techniques selected from the group comprising,electroplating, evaporation, ink deposition and sputtering.Electroplating is especially attractive to practice this invention.Accordingly, an electroplating technique may be used to: i) electroplatean (In,Ga)Se compound layer on a contact layer (such as Mo, Ru, Ir etc.)deposited on a substrate (such as glass, kapton, metallic foil etc.),and ii) electroplate a metal film over the (In,Ga)Se layer. In this casemetal film may comprise Cu, In and Ga. For example, the metal film maybe obtained by electroplating discrete layers of Cu, Ga and In, or byelectroplating a metallic Cu—In—Ga alloy, or by electroplating a binaryalloy layer (such as a Cu—Ga, In—Ga or Cu—In alloy) and a discrete layerof Cu or In or Ga.

Alternately, precursor stack preparation may include; i)electrodeposition of a Cu layer on the contact layer, ii)electrodeposition of an (In,Ga)Se compound layer over the Cu layer, andiii) electroplating a metal film over the (In,Ga)Se layer. In this case,the metal film may contain just In and Ga (such as an electroplatedIn—Ga alloy, an electroplated In/Ga or Ga/In or Ga/In/Ga etc. stack), orit may also contain additional Cu like the case described above.

Once the electroplated precursor film or stack is formed on the base,selenium may be deposited on the precursor by evaporation orelectroplating. A dopant such as Na may also be added to the structureformed. The structure may be heated to a temperature in the range of400-600° C. to form a CIGS layer. The Ga distribution in the CIGS layermay be any of the cases depicted in FIG. 5 depending on the Ga contentof the (In,Ga)Se layer and the metal film.

Solar cells may be fabricated on the CIGS compound layers of the presentinvention using materials and methods well known in the field. Forexample a thin (<0.1 microns) CdS layer may be deposited on the surfaceof the compound layer using the chemical dip method. A transparentwindow of a transparent conductive oxide such as ZnO may be depositedover the CdS layer using MOCVD or sputtering techniques. A metallicfinger pattern is optionally deposited over the ZnO to complete thesolar cell.

Although the present invention is described with respect to certainpreferred embodiments, modifications thereto will be apparent to thoseskilled in the art.

1. A process of forming a Group IBIIIAVIA absorber on a base,comprising: forming a first layer comprising an indium-gallium-selenidecompound film on the base; forming a metallic layer on the first layer,the metallic layer comprising a Group IB metal, a Group IIIA metal andanother Group IIIA metal without a Group VIA material, wherein the baseis at a substantially ambient temperature when the metallic layer isformed, and wherein the first layer and the metallic layer are distinctlayers with no substantial reaction therebetween during the forming ofthe metallic layer; and reacting the first layer, the metallic layer anda Group VIA material.
 2. The process of claim 1, wherein the first layerfurther comprises a first metal film of a Group IB metal, wherein theindium-gallium-selenide compound material film is deposited over thefirst metal film at a substantially ambient temperature.
 3. The processof claim 1, wherein forming the metallic layer comprises: depositing acopper film onto the first layer; depositing a gallium film onto thecopper film; and depositing an indium film onto the gallium film.
 4. Theprocess of claim 3, wherein forming the metallic layer further comprisesdepositing another copper film onto the indium film.
 5. The process ofclaim 1, wherein the gallium to indium molar ratio of theindium-gallium-selenide compound film is in the range of 0 to 0.8. 6.The process of claim 1, wherein a molar ratio of gallium to indium inthe metallic layer is in the range of 0.2 to 0.3.
 7. The process ofclaim 1, wherein the step of reacting comprises depositing a Group VIAmaterial on the metallic layer, thereby forming a pre-absorberstructure, and heating the pre-absorber structure 200-600° C.
 8. Theprocess of claim 2, wherein the step of forming the first layercomprises electrodepositing a copper film on the base andelectrodepositing the (In,Ga)Se compound film on the copper film.
 9. Theprocess of claim 1, wherein the step of forming the first layercomprises electrodepositing the (In,Ga)Se compound film on the base. 10.The process of claim 9, wherein forming the metallic layer comprises:electrodepositing a copper film onto the (In, Ga) Se compound film;electrodepositing a gallium film onto the copper film; andelectrodepositing an indium film onto the gallium film.
 11. The processof claim 10, wherein forming the metallic layer further compriseselectrodepositing another copper film onto the indium film.
 12. Theprocess of claim 8, wherein forming the metallic layer compriseelectrodepositing a copper-indium-Gallium alloy film onto the (In, Ga)Se compound film.